Epilepsy is the second most frequent chronic condition neurologists encounter in their day-to-day practice. Suboptimal response to antiepileptic drugs may be due to numerous factors, some of which may be modified. For patients who remain nonresponsive to various combinations of drugs and doses, the therapeutic options include surgery, or vagal nerve stimulation, yet not all patients with drug-resistant epilepsy are eligible for the procedures. Ketogenic diet is one of the available non-pharmacological options in such cases. Many years of experience, dating back to the 1920s, have allowed to identify disorders in which ketogenic diet is the treatment modality of choice. In the case of broadly defined drug-resistant epilepsy in children, and increasingly often in adult patients, ketogenic diet is an established adjunctive treatment. Most of the available data concerns the paediatric population. This paper is a review of literature data regarding ketogenic diet for drug-resistant epilepsy. The diet’s application has been discussed for the management of various neurological disorders such as glucose transporter type 1 deficiency syndrome (GLUT1 deficiency syndrome), Dravet syndrome, tuberous sclerosis, Doose syndrome, Lennox–Gastaut syndrome, Ohtahara syndrome, Landau–Kleffner syndrome and epileptic encephalopathy with continuous spike and wave during sleep. The literature data demonstrate ketogenic diet to be an effective nonpharmacological treatment method. Importantly, however, in many cases instead of being considered a therapy of last resort, it should be offered earlier in the management.